Systems and methods for varying elastic modulus appliances

ABSTRACT

The present invention provides improved devices, systems and methods for repositioning teeth from an initial tooth arrangement to a final tooth arrangement. Repositioning is accomplished with a system comprising a series of polymeric shell appliances configured to receive the teeth and incrementally reposition individual teeth in a series of successive steps. The individual appliances may be formed from layers having different stiffnesses (elastic moduluses), and the stiffnesses of successive appliances may be different, or both.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This Application is a continuation-in-part of application Ser.No. 09/616,830, filed Jul. 14, 2000, which claims the benefit of priorprovisional application Ser. Nos. 60/199,650 and 60/199,649, both filedon Apr. 25, 2000, the full disclosures of which are incorporated hereinby reference. The disclosure of this application is related toapplication Ser. No. 09/616,222, filed on the same day, the fulldisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] This invention relates in general to a method of repositioningteeth for use in orthodontic treatment. Particularly, this inventionrelates to the use of orthodontic appliances for producing toothmovements. More particularly, this invention relates to the use of aplurality of elastic repositioning appliances for producing such toothmovements.

[0003] Orthodontic treatments involve repositioning misaligned teeth andimproving bite configurations for improved cosmetic appearance anddental function. Repositioning teeth is accomplished by applyingcontrolled forces to the teeth over an extended period of time. This isconventionally accomplished by wearing what are commonly referred to as“braces.” Braces comprise a variety of appliances such as brackets,bands, archwires, ligatures, and O-rings. After they are bonded to theteeth, periodic meetings with the orthodontist are required to adjustthe braces. This involves installing different archwires havingdifferent force-inducing properties or by replacing or tighteningexisting ligatures. Between meetings, the patient may be required towear supplementary appliances, such as elastic bands or headgear, tosupply additional or extraoral forces.

[0004] Although conventional braces are effective, they are often atedious and time consuming process requiring many visits to theorthodontists office. Moreover, from a patient's perspective, they areunsightly and uncomfortable. Consequently, alternative orthodontictreatments have developed. A particularly promising approach relies onthe use of elastic positioning appliances for realigning teeth. Suchappliances comprise a thin shell of elastic material that generallyconforms to a patient's teeth but is slightly out of alignment with theinitial tooth configuration. Placement of the elastic positioner overthe teeth applies controlled forces in specific locations to graduallymove the teeth into the new configuration. Repetition of this processwith successive appliances comprising new configurations eventually movethe teeth through a series of intermediate configurations to a finaldesired configuration. A full description of an exemplary elasticpolymeric positioning appliance is described in U.S. Pat. No. 5,975,893,and in published PCT application WO 98/58596 which designates the UnitedStates and which is assigned to the assignee of the present invention.Both documents are incorporated by reference for all purposes.

[0005] In addition to their ease of use, polymeric positioningappliances are generally transparent, providing an improved cosmeticappearance, and impart substantial force on the teeth, due to stiffnessof the appliance. The stiffness of an elastic positioning appliance is aresult of the modulus of the thermoformable polymer materials from whichit is made. The higher the modulus of the materials, the higher thestiffness of the appliance. When a patient positions such an applianceover a prescribed group of teeth, one or more of the teeth will providea base or anchor region for holding the positioning appliance in placewhile the stiffness of the polymeric material will impart a resilientrepositioning force against one or a portion of the remaining teeth.However, the stiffer the appliance, the more difficult it is to slip themisaligned appliance over the teeth and fully engage the appropriatesurfaces; the appliance often has the tendency to disengage or “popoff”. Likewise, once it is firmly seated, it is more difficult toremove. Further, a stiff appliance is less forgiving in cases of loweredpatient compliance. If a patient were to remove the appliance for anunprescribed period of treatment time, the patient's teeth may moveslightly out of the planned tooth arrangement. When attempting toreapply the appliance, it may be too rigid to accommodate these slightdifferences and a new appliance may need to be created. Similarly, thetooth positions defined by the cavities in each successive appliancemust not differ beyond a limiting dimension from those defined by theprior appliance or, again, it may be too rigid to accommodate thedifferences. Consequently, only small increments in tooth repositioningmay be made with each appliance.

[0006] Thus, it would be desirable to provide tooth positioners,systems, and methods which apply adequate force to selected teeth yetovercome the inherent limitations of stiffness in the polymericmaterial. Likewise, it would be desirable to reduce the number ofpositioners required for a treatment plan by increasing the size of therepositioning increments throughout the plan. Further, it would bedesirable to reduce the cost of lowered patient compliance by reducingthe need for new appliances to be created for patient treatmentresumption. At least some of these objectives will be met by the designsand methods of the present invention described hereinafter.

SUMMARY OF THE INVENTION

[0007] The present invention provides improved devices, systems andmethods for repositioning teeth from an initial tooth arrangement to afinal tooth arrangement. Repositioning is accomplished with a systemcomprising a series of polymeric appliances configured to receive theteeth in a cavity and incrementally reposition individual teeth in aseries of successive steps. This is accomplished by applying force tospecific surfaces of the teeth to cause directed movement. In order toapply such force, one or more of the teeth will provide a base or anchorregion for holding the positioning appliance in place while thestiffness of the polymeric material will impart a resilientrepositioning force against one or more of the remaining teeth. However,such stiffness creates limitations in ease of use, patient compliance,and overall cost in material, manufacturing labor and treatment time.

[0008] To overcome these limitations, the present invention utilizespolymeric or other material appliances with portions differing inrigidity, hardness, or stiffness. Portions of the appliance designed toapply specific forces may have different elastic moduluses (stiffnesses)and/or hardnesses than other portions. Alternatively, elastic modulusesand/or hardnesses may vary from one appliance to the next in asuccessive series to accomplish various treatment goals. Thus, thesystems and methods of the present invention provide the design,production and use of such multiple stiffness positioning appliances inorthodontic treatment. Similarly, the devices of the present inventionprovide variable stiffness appliances which may be used independentlyfor purposes other than repositioning, such as for retaining teeth in adesired position. Thus, reference hereinafter to repositioningappliances with portions having differing or varying stiffnesses orhardnesses is not intended to limit the scope of the present inventionand is understood to include appliances of the described design forother purposes.

[0009] In a first aspect of the present invention, an elasticrepositioning appliance may be comprised of portions with differingelastic moduluses. Elastic modulus may be used to express or describethe stiffness of a material or a material's resistance to elasticdeformation. Therefore, elastic modulus may be used hereinafter to referto stiffness. The different portions of the appliances will also usuallyvary in hardness. More usually, stiffer portions will be harder whilethe less stiff portions will be softer. Hardness is usually measured asa “durometer” reading on either the A or the D scale. In most instances,the present invention will be more concerned with the elastic modulus ofthe material since that will effect the force applied to the teeth foreither moving the teeth or for gripping or anchoring the teeth. In otherinstances, however, the hardness of the material may be more important,e.g., to avoid trauma to soft tissue regions engaged by the appliance.The remaining description and claims generally refer to materials havinggreater and lesser stiffnesses. It will be appreciated that suchterminology will also comprise materials having greater and lesserhardnesses.

[0010] The elastic modulus of a material is the ratio of the incrementof unit stress to an increment of unit deformation within the elasticlimit. When a material is deformed within the elastic limit, the bondsbetween adjacent atoms are stretched but not broken. The magnitude ofthe elastic modulus is indicative of the atomic and molecular bondingforces. When the stress is relieved, the material returns to itsoriginal shape and the deformation is nonpermanent. Different materialsmay have different elastic moduluses based on their molecularstructures. Some materials, such as certain polymers, may be speciallyproduced to have different elastic moduluses while retaining similarchemical compositions (and thus assuring compatibility of the differentmodulus materials in a single structure). Likewise, the elastic modulusof a polymer or other material may be enhanced or otherwise modified.This may be achieved by adding a powder, such as CaCO₃, talc, TiO₂,glass, diamond or a polymer powder, to name a few. In addition, this maybe achieved by embedding structural reinforcements, such as metalpieces, strips, wires, mesh, lattices, networks, polymeric filaments, orthe like. In addition, the elastic modulus may be altered bypost-production methods, such as layering, coating, interpenetrating,treating with various chemical agents, and altering the temperature, toname a few. In the resulting appliance, the elastic moduluses of thevarying portions will usually range from 0.5 to 5 GigaPascal (GPa),although in some instances portions of the appliance may fall outside ofthis range. The elastic modulus of one portion may differ from anotherportion by 25% to 600%, or more.

[0011] The differing elastic moduluses of different portions of thedental appliance shells of the present invention will exist while thedevice is present over teeth in a normal oral environment. Thus,different portions of the appliance shell will impart different forcesto the immediately underlying teeth, where the level of the forcedepends both on the device geometry or tooth positions (relative to theunderlying tooth or teeth, which may vary over time) and on the elasticmodulus of that portion of the device (which will remain constant overtime in the normal oral environment). The present invention should bedistinguished from that described in copending application Ser. No.09/250,962, where the stiffness of a dental appliance shell may changeover time by expose to a non-oral environment, such as elevatedtemperature or changed osmolality. Of course, the dental applianceshells of the present invention which have different portions withdiffering stiffness may also incorporate regions (including the entireappliance) where a change in stiffness may be induced according to theteachings of application Ser. No. 09/250,962, the full disclosure ofwhich is incorporated herein by reference.

[0012] In a first embodiment, portions of the shell of an elasticrepositioning appliance may be composed of material(s) which differ inelastic moduluses and/or hardnesses along a mesial-distal axis. Amesial-distal axis may be defined as an axis following the gingival lineor dental arch. Thus, the elastic repositioning appliance may becomprised of portions with a lower elastic modulus covering the molars,for example, and portions with a higher elastic modulus covering theremainder of the teeth. In this example, the portions may be relativelylarge so that a portion may receive one or more teeth, such ascontiguous molars. This may be utilized when one or more teeth are toprovide an anchor or base region for imparting repositioning forceagainst another tooth or teeth. The portion of the appliance coveringthe anchor teeth may be of a relatively flexible nature with a lowerelastic modulus than the portion covering the teeth to be repositioned.This is because the portions covering the anchor teeth may not need toapply repositioning forces to the teeth they cover; they may merely bedesigned to hold the appliance in place. Consequently, a high level ofrigidity or stiffness may not be required. However, it may beappreciated that portions covering anchor teeth may in fact require ahigher stiffness material than other portions, including portions whichare designed to apply repositioning forces. Thus, any variation ofstiffness or elastic modulus along a mesial-distal axis is included inthis embodiment.

[0013] The introduction of such portions or regions with moreflexibility provides utility in ease of use for the patient. The patientmay find ease in positioning the appliance with the more flexibleportions first which may guide the appliance in placement of the morerigid, slightly misfit portions designed for repositioning. Thissequence may be reversed in removal of the appliance. Likewise, suchflexibility may also allow for any slight differences in mold versusappliance versus dentition geometry which may otherwise make placementand removal of the appliance more difficult. In some cases, a generallymisfit appliance may “pop off” or have a tendency to disengage even whenproperly positioned over the teeth. Increased flexibility may reducethese tendencies.

[0014] In further embodiments, portions of the elastic repositioningappliance may vary in elastic moduluses along different and/oradditional axes. For example, moduluses may vary along a facial-lingualaxis. Facial may be defined as next to or toward the lips or cheek,including terms labial and buccal. Lingual may be defined as next to ortoward the tongue. Thus, a facial-lingual axis may be described as anaxis following a radial or similar line from the tongue toward the lipsor cheek and vice versa. Likewise, moduluses may vary along agingival-crown axis. This may be described as a substantially verticalaxis following a line from the top of the crown at the edge of theocclusal surface of a tooth toward the gingival line or root and viceversa. In a preferred embodiment, an appliance may have a portion with alower elastic modulus covering the occlusal surfaces of the teeth and aportion with a higher elastic modulus covering the remaining surfaces ofthe teeth. Thus, the moduluses may vary along a facial-lingual axisand/or a gingival-crown axis, depending on the boundaries of thedelineated portions. Such a design may incorporate added flexibility tothe appliance while maintaining adequate repositioning forces in themost efficient areas.

[0015] In addition to varying in stiffness along the axes describedabove, the appliances of the present invention may vary in stiffness orhardness over the “thickness” of the appliance. Usually, such variationsand stiffness over the thickness will be accomplished by layering thedevice, i.e., with layers of differing stiffnesses or hardnesses beingplaced successively over the mold used to form the appliances, asdescribed in more detail below. Thus, the appliances may comprise shellshaving first and second portions, as generally described above, whereeach of those portions comprise layers in a laminar structure. Usually,at least one of the first and second portions will comprise a continuouslayer along the mesial-distal axis. The second and optionally additionallayers may also be continuous along the mesial-distal axis, but willoften be discontinuous, i.e., broken into two or more segments. Suchlayered devices can provide a variety of benefits. For example, layersformed from stiffer or harder materials can be used to more firmlyengage teeth, while the less stiff or softer layers can be used toprovide compliance and greater elasticity. In a particular preferredembodiment, the appliance comprises a discontinuous inner layer and acontinuous outer layer. At least a portion of the inner layer isconfigured to engage individual teeth or groups of teeth and will bestiffer or harder than the outer layer. The outer layer, which is lessstiff and therefore more compliant, provides the elasticity to move theteeth relative to one another, while the harder inner layer firmlyengages the teeth to provide a better grip or anchor upon the teeth.

[0016] It may be appreciated that the elastic modulus of the applianceshells may vary over any number of delineated portions. Such portionsmay be of any size, shape, thickness, or dimension. Thus, such portionsmay receive entire teeth or they may be of the size to cover only aportion of a tooth or dental surface. When portions are relativelylarge, an appliance may be divided into, for example, two to fiveportions. Portions adjacent to one another differ in elastic moduluses,however not all portions of an appliance may differ from each other,such as in the case of an appliance with portions alternating betweentwo moduluses. When portions are relatively small, an appliance maycontain an unlimited number of portions, varying along any axis orcombination of axes.

[0017] In a second aspect of the present invention, such appliancescomprised of portions having differing stiffness may be usedindependently or in a series with similar or differing devices. Whenused independently, the appliance may be worn to achieve a specific goalwith a single device. For example, the appliance may be used as a“retainer” to hold the teeth in a desired position. Or, the appliancemay be used for a specific one-time repositioning movement, such as“finishing” or correcting a slight misalignment. When used in a series,the appliances may comprise a system for repositioning teeth from aninitial tooth arrangement to a final tooth arrangement. In this case, aplurality of incremental elastic position adjustment appliancescomprising polymeric or other material shells are successively worn by apatient to move teeth from one arrangement to a successive arrangement.Individual appliances may be configured so that their tooth-receivingcavity has a shape or geometry corresponding to an intermediate or endtooth arrangement intended for that appliance. Thus, successiveindividual appliances may have a shape or geometry differing from thatof the immediately prior appliance. According to the present invention,some or all of the individual appliances may also be comprised of amaterial stiffness differing from the stiffness of the immediately priorappliance. In addition, each individual appliance be comprised ofportions with varying stiffnesses. In some cases, of course, individualappliances in the system may not vary in stiffness from prior orsuccessive appliances, but only in geometry. In other cases, individualappliances may vary only in stiffness (and not in geometry) whencompared to immediately prior or subsequent appliances. Thus, systemsaccording to the present invention may be comprised of appliances havingstiffness varying within the appliance and/or from one appliance to thenext in the series.

[0018] In a specific embodiment, a system of elastic repositioningappliances may comprise individual appliances having uniform elasticmoduluses over their entire tooth contact area where the moduluses willdiffer among successive appliances used in a course of treatment. Theelastic modulus of a given appliance may be chosen to be most suitablefor a specific type of tooth movement, such as translating, tipping,root uprighting, rotation, extrusion, intrusion or a combination ofthese. For example, translation may require 70-120 gm of force, whereasrotation may only require 35-60 gm of force. Therefore, an elasticpositioning appliance designed for translating teeth may need to have ahigher elastic modulus than one designed for purely rotating teeth. Thisis again due to the fact that stiffness of the appliance is a criticalfactor in imparting repositioning force. Consequently, a series ofappliances may be produced for a treatment plan in which successiveappliances designed for a specific tooth movement may all havesubstantially similar elastic moduluses. At the point in the treatmentplan in which a different type of tooth movement is desired, furtherappliances designed for the new tooth movement may have substantiallysimilar elastic moduluses to each other but different from the previousappliances. Such a sequence may be repeated at any time or may continuewith new moduluses and tooth movements.

[0019] In an additional specific embodiment, one or more appliances maybe produced with a suitably flexible elastic modulus to receive andresiliently reposition teeth from an unprescribed arrangement to aprescribed arrangement. This might be necessary in cases of loweredpatient compliance. If a patient were to remove an appliance for anunintended and/or extended period of a prescribed treatment time, thepatient's teeth may move slightly out of the planned tooth progression.When attempting to reapply the appliance, an appliance which is toorigid may not be able to accommodate these slight differences. Thus, amore flexible appliance (but having an identical geometry) may beproduced for this purpose and may be incorporated into the treatmentplan at any given point in the series of successive appliances. Theability to return to the same geometry is an advantage because itminimizes the need to replan the treatment protocol.

[0020] In a third aspect of the present invention, systems forrepositioning teeth from an initial tooth arrangement to a successivetooth arrangement comprise a plurality of incremental elastic positionadjustment appliances in which at least one appliance has the same shapeyet different elastic modulus as an immediately prior appliance. In aspecific embodiment, a series of incremental appliances may be producedwith differing elastic moduluses to reposition teeth from an initialtooth arrangement to the next successive tooth arrangement in aprogression of arrangements to the final arrangement. Each of theappliances in the series from the first to the next successive tootharrangement may have the same shape or geometry since the tooth movementrepresents one step in tooth movement. However, the variance in elasticmoduluses may allow for a larger step or increment in tooth movementthan may be obtainable with consistent, rigid appliances. For example,an appliance may be produced with a tooth arrangement which issubstantially misaligned from the initial arrangement. High modulusappliances may not be flexible enough to allow the appliance to fit overthe teeth in the initial arrangement. However, a series of appliances ofthe same shape may be produced with increasing elastic moduluses fromrelatively low to adequately high. The patient may begin with the lowestelastic modulus appliance which may be the most flexible to fit over theteeth. As the teeth are repositioned, the patient may successivelyutilize each appliance in increasing modulus until the teeth haveconformed to the successive tooth arrangement. At that time, the patientmay begin a new series of appliances with varying moduluses and a shapeto reposition the teeth to the arrangement of the next step in therepositioning progression. The ability to reduce the number of differentappliance geometries required for a single course of treatment canprovide a significant reduction in planning effort and manufacturingcosts.

[0021] In a fourth aspect of the present invention, the elastic modulusof an appliance or portions of an appliance may be modified in a numberof different ways. To begin with, the elastic modulus may be determinedby the choice of materials. For example, metals will generally have ahigher elastic modulus than polymers due to atomic structure. Forexample, the modulus values for metals may range between 48 and 414 GPa,whereas the modulus for polymers may range from 0.5 to 35 GPa. Thus, itwill be possible to form appliances having moduluses which differgreatly by forming different portions from metal(s) and polymer(s), orby forming successive appliances from metals and polymers. Usually,however, the appliances will comprise or consist of a polymeric shellformed from a single polymer, multiple polymers, copolymers, and thelike, typically by thermoforming and/or lamination. Stiffness of apolymer may be varied within a range (typically 0.5 GPa to 5 GPa) bychanging the molecular structure of the polymer chains. Polymer chainswith hindered side-chains are unable to pack as closely as those withsmaller side-chains. Thus, such a polymer may have more intermolecularmotion and therefore a lower bulk elastic modulus. Stiffness can also bechanged by controlling the degree of cross-linking as well as thecross-linking entity within a polymer or copolymer. Further,alternatively, differing elastic moduluses may be created within thesame polymer shell by layering or laminating the same or differentpolymers. Two layers of a polymer material bonded together to form anintegral appliance, i.e., an appliance having a monolithic shellstructure where the layers are resistant to delamination, may have ahigher elastic modulus than a single layer of such material. Thirdly,different elastic moduluses may be created with a single layer of onetype of polymer material by production methods, such as coating,treating with various chemical agents, and altering the temperature, toname a few.

[0022] Further, different elastic moduluses may be produced by formingselectively reinforced and/or composite-type materials. For example, apolymer material may be reinforced with structures such as strips,wires, pieces, mesh, lattices, networks, and the like. These structuresmay be comprised of any suitable material, particularly metals andalloys but also including polymer filaments, wires, braids, and thelike. Likewise, composite materials may be comprised of interpenetratingpolymeric networks. An interpenetrating polymeric network is comprisedof a base material and an additional material that interpenetrates thebase material to alter its mechanical properties. For example, the basematerial (A) may be a solid polycarbonate. The added material (B) may bea liquid polymer, monomer or crosslinking agent which is allowed tointerpenetrate and activate to form a composite network. The composite(A+B) may have a stiffness which is greater than the sum of its parts,(A) and (B). Further, another material (C) may also be allowed tointerpenetrate and activate to form a new composite network. Thecomposite (A+B+C) may also have a stiffness which is greater than thesum of its parts, (A), (B) and (C). With this method, any number ofcomposites may be formed providing a wide range of mechanicalproperties, specifically stiffnesses. In addition, a number of theseproduction methods may provide materials with gradual changes in elasticmoduluses. For example, purposely irregular coating of a polymermaterial may provide higher stiffness in areas with thicker coating andlower stiffness in areas with thinner coating. This may be applied to anumber of production methods.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a perspective illustration of an embodiment of anappliance of the present invention and descriptive axes.

[0024]FIG. 2 illustrates an embodiment of an appliance with relativelylarge portions varying in elastic modulus along a mesial-distal axis.

[0025]FIG. 3 illustrates an embodiment of an appliance with smallerportions varying in elastic modulus in a non-symmetric pattern along amesial-distal axis.

[0026]FIG. 4 illustrates an embodiment of an appliance varying inelastic modulus along a mesial-distal axis in which portions coveringproximal or interproximal spaces are of differing modulus.

[0027]FIG. 5 illustrates a variety of appliance portions varying inelastic modulus along a mesial-distal axis.

[0028]FIG. 6 is a perspective illustration of an embodiment of anappliance varying in elastic modulus along a facial-lingual axis.

[0029]FIG. 7 illustrates a variety of appliance portions varying inelastic modulus along a facial-lingual axis.

[0030]FIG. 8 is a perspective illustration of an embodiment of anappliance varying in elastic modulus along a gingival-crown axis.

[0031]FIG. 9 illustrates a variety of appliance portions varying inelastic modulus along a gingival-crown axis.

[0032]FIG. 10 illustrates a variety of appliance portions varying inelastic modulus along one or more described axes.

[0033]FIG. 11 depicts a series of appliances differing in elasticmodulus at specific intervals throughout a treatment plan.

[0034]FIG. 12 illustrates the use of an “off track” appliance in aprescribed treatment plan.

[0035]FIG. 13 illustrates the use of a series of appliances withgradually increasing elastic moduluses and similar or identical geometryin a series of intervals throughout a treatment plan.

[0036]FIG. 14 illustrates a method of fabricating a multi-modulusappliance.

[0037]FIG. 15 illustrates a method of layering to fabricate amulti-modulus appliance.

[0038]FIG. 16 illustrates an additional method of layering to fabricatea multimodulus appliance.

[0039]FIG. 17 illustrates an exemplary layered appliance according tothe present invention with portions broken away.

[0040]FIG. 18 is a cross-sectional view taken along line 18-18 of FIG.17.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0041] The present invention provides improved devices, systems andmethods for incrementally repositioning teeth using a plurality ofdiscrete polymeric appliances of variable flexibility, where eachappliance successively repositions one or more of the patient's teeth byrelatively small amounts. Flexibility may be defined by elastic modulusof the polymeric material and may vary within a given appliance or mayvary throughout a series of appliances according to a prescribedorthodontic treatment plan.

[0042] Referring to FIG. 1, portions of an elastic repositioningappliance 100 may vary in elastic modulus along a mesial-distal axis101, facial-lingual axis 102, gingival-crown axis 103, or any axisin-between these representative axes. As previously described, amesial-distal axis may be described as an axis following the gingivalline or dental arch, a facial-lingual axis may be described as an axisfollowing a radial or similar line from the tongue area toward the lipor cheek area, and a gingival-crown axis may be described as an axisfollowing a substantially vertical line from the crown of a tooth towardthe gingival line or root. Such axes are defined for descriptivepurposes and are not intended to limit the scope of the presentinvention.

[0043] As shown in FIG. 2, portions of an elastic repositioningappliance 100 may vary in elastic modulus mesial-distally. Forillustrative purposes, portions with a lower elastic modulus 110 areshaded to easily distinguish differences in elastic modulus throughout adevice. In this example, the appliance 100 may be described as havingthree portions. Two portions cover contiguous sets of molars and arecomprised of an elastomeric material of a lower elastic modulus 110 andare thus shaded. The portion in-between these portions is of a higherelastic modulus 111 and is thus not shaded. All portions in thisembodiment are relatively large so that the portions may receive one ormore teeth, such as molars, premolars, incisors, and the like. Likewise,nonadjacent portions may have the same elastic modulus, such as the twolower elastic modulus 110 portions, or they may be different from eachother while maintaining a difference from the higher elastic modulus 111portion. In other words, an appliance 100 with three distinct portionsmay be comprised of two or three elastic moduluses.

[0044] As illustrated in FIG. 3, such portions may not be symmetricaland they may not cover more than one tooth. Portions with a lowerelastic modulus 110 may alternate in an uneven fashion along amesial-distal axis as shown. In addition, adjacent portions may be of asize to cover only a portion of a tooth or dental surface. Referring toFIG. 4, portions of lower elastic modulus 1 10 may be present coveringthe facial or lingual surfaces of the teeth, while portions of higherelastic modulus 111 may be present covering the proximal orinterproximal spaces. This may be advantageous to provide repositioningforces, such as translation forces, at the most efficient locations forthis type of movement. At the same time, flexibility is provided inportions that may be less involved in the application of force, allowingmore freedom and comfort for the patient.

[0045] Referring to FIG. 5, the elastic modulus of an appliance 100 mayvary over any number of delineated portions and may be of any size,shape, thickness or dimension, to name a few. Such portions may be sizedto receive an entire tooth 115 or they may be of the size to cover onlya portion of a tooth. For example, a portion with a lower elasticmodulus 110 may be sized to cover a proximal or interproximal space 116,including portions covering the gingival line. This may be desirable toprovide comfort to the gums when wearing the appliance, and also toincrease the contact of the appliance with the interproximal regions. Inthis case, softer, more flexible material may be able to form moreclosely to the interproximal regions, enabling a higher level ofrepositioning force to be applied.

[0046] Portions may also be sized and arranged to cover part of a facialsurface 117, or two or more of such portions may cover part of a facialsurface 118, allowing the elastic modulus to vary mesial-distally withina single tooth. Further, the portion may be sized to cover an isolatedregion of a facial surface 119. Such embodiments are a limitedpresentation of the possible sizes, configurations, and combinations ofvarying elastic modulus portions in an appliance 100 of the presentinvention. Such possibilities may be unlimited.

[0047] As shown in FIG. 6, portions of an elastic repositioningappliance 100 may vary in elastic modulus facial-lingually. In thisembodiment, the appliance 100 is shown to have a portion with a lowerelastic modulus 110 covering a portion of the occlusal surfaces of theteeth and a portion with a higher elastic modulus 111 covering theremaining surfaces of the teeth. Thus, the elastic modulus varies alonga facial-lingual axis. Such a design may be beneficial to providerepositioning forces, such as translation forces, along the proximaland/or interproximal spaces which are the most efficient locations forthis type of movement. At the same time, flexibility is provided inportions that may be less involved in the application of force, theocclusal surfaces. This may allow increased freedom and comfort for thepatient while maintaining adequate repositioning forces.

[0048] Referring to FIG. 7, the elastic modulus of an appliance 100 mayagain vary over any number of delineated portions and may be of anysize, shape, thickness or dimension, to name a few. A portion of lowerelastic modulus 110 may be sized to cover only a portion of a tooth. Forexample, it may cover the center of the occlusal surface 125,alternating cusps or cusp tips 126, or isolated portions of any givencusp or cusp tip 127. Likewise, a portion of lower elastic modulus 110may be of a larger size to cover, for example, the outside margin orbuccal cusps of a tooth 128 or the inside margin or lingual cusps of acontiguous grouping of teeth 129. As before, such embodiments are alimited presentation of the possible sizes, configurations, andcombinations of varying elastic modulus portions in an appliance 100 ofthe present invention. Such possibilities may be unlimited.

[0049] As shown in FIG. 8, portions of an elastic repositioningappliance 100 may vary in elastic modulus crown-gingivally. In thisembodiment, the appliance 100 is shown to have a portion with a lowerelastic modulus 110 covering the occlusal surfaces of the teeth and aportion with a higher elastic modulus 111 covering the remainingsurfaces of the teeth. This is a modified representation of theembodiment depicted in FIG. 6 in which the portion of lower elasticmodulus 110 partially covered a portion of the occlusal surfaces. Inthis embodiment, the occlusal surface is substantially covered with thelower elastic modulus 110 material, therefore it may be considered to beuniform, non-variable, along a facial-lingual axis. It may be moreproperly described as varying along a gingival-crown axis, as the lowerelastic modulus 110 portion may extend over the cusps of the toothcrowns. Thus, the lower elastic modulus 110 material may be seen asbeing located at the tip of the crown region and vary to a higherelastic modulus 111 material toward the gingival line or margin. Inaddition, a higher elastic modulus 111 material along the gingival lineor margin may improve retention of the device on the teeth. This mayalso reduce the need for attachment devices to aid in retention. A fulldescription of exemplary attachment devices and methods for a dentalappliance is described in co-pending application Ser. No. 09/454,278,incorporated by reference for all purposes and assigned to the assigneeof the present inventor. However, such modulus differences are presentedonly for descriptive purposes and such portions may vary over one ormany axes simultaneously or in isolated regions of an appliance 100.

[0050] It may be appreciated that the advantages offered by a lowerelastic modulus along the occlusional surfaces, as depicted in FIG. 6and FIG. 8, may be further increased by removing the material from theshell in these areas. Removal of material may form a window such thatwhen the shell is positioned over the patient's teeth, portions of theteeth beneath the window may be exposed. In a preferred embodiment, apolymeric shell may have a plurality of windows over portions of theocclusal surfaces of the teeth. In this case, segments of the shell maystill be present along the facial and lingual surfaces of the teeth andacross the interdental regions or spaces between the teeth. Exposure ofthe occlusal surfaces in appropriate size and location may allowinterdigitation of the upper and lower teeth. This may also be achievedwith the presence of one or a few larger windows over portions of theocclusal surfaces of the teeth. In these cases, segments of the shellmay not be present across the interdental regions or spaces between theteeth. In either case, interdigitation of at least portions of the upperand lower teeth may benefit tooth and jaw orientations, leading toimproved treatment, appearance, comfort and consequently patientcompliance. Thus, such windows may provide the benefits offered by alower elastic modulus, such that the lowest stiffness may be provided bythe absence of the material, while providing additional benefitsdescribed above. A full description is provided in co-pendingapplication [Attorney Docket No. 018563-001510], assigned to theassignee of the present invention.

[0051] Referring to FIG. 9, the elastic modulus of an appliance 100 mayagain vary over any number of delineated portions and may be of anysize, shape, thickness or dimension, to name a few. A portion of lowerelastic modulus 110 may be sized to cover only a portion of a toothalong this axis. For example, it may cover the upper portion of thelingual surfaces near the cusps of the crown 135, or a midway “stripe”through the lingual surface of a tooth 136. Likewise, it may be sized sothat more than one “stripe” may cover the surface of a tooth 137, as inthe case of a portion at the gingival margin and a portion near thecusps of the crown. Similarly, a portion of lower modulus may be sizedso that it covers a contiguous grouping of teeth, such as the buccalsurfaces along the gingival margin 138. As before, such embodiments area limited presentation of the possible sizes, configurations, andcombinations of varying elastic modulus portions in an appliance 100 ofthe present invention. Such possibilities may be unlimited.

[0052] As illustrated in FIG. 10, variance in elastic modulus inrelation to size, shape, location, orientation, and axis, as describedabove, may be combined in a single appliance 100 to provide an unlimitedvariety of appliance 100 designs and constructions. In this example,portions of the appliance 100 vary mesial-distally, such as by comparingthe lower elastic modulus 110 portion covering a group of incisors withthe higher elastic modulus 111 portion covering the canine tooth.Portions may also vary facial-lingually, as depicted by the partialcovering of the occlusal surfaces of the molars 150 or the isolatedportion of a given cusp or cusp tip 127. Likewise, portions may varycrown-gingivally, such as the portions covering the buccal surfacesalong the gingival margin 138. These portions also vary mesial-distallycreating a compound variance, as portions adjacent to these areas arenot identical, as in comparison of portions covering the buccal surfacesalong the gingival margin 138 with the adjacent tooth 151 having partialcovering of the occlusal surface of the molar 150. Portions may alsovary along the three major axes simultaneously. This can be seen in theportion covering the center of an occlusal surface 125, which variesfacial-lingually, the lingual face of a molar along the gingival margin152, which varies crown-gingivally, and the adjacent uniformly coveredmolar 153, which varies mesial-distally in relation to the previousportions.

[0053] According to the present invention, systems for repositioningteeth from an initial tooth arrangement to a final tooth arrangement maybe comprised of a plurality of incremental elastic position adjustmentappliances with varying elastic moduluses. Thus, in addition to combinedvariances in a given appliance 100, as described above, a plurality ofsuch appliances 100 with differing patterns of elastic modulus variancemay be used in a system for repositioning teeth throughout a sequence oftooth arrangements. This may be illustrated by FIGS. 2-10 with differingtooth geometries, viewed as a series of appliances 100 for a singletreatment plan.

[0054] Alternatively, as shown in FIG. 11, the polymeric shells of theappliances 100 may have uniform elastic moduluses over their entiretooth contact area. In this depiction, each appliance 100 differs inshape or tooth geometry and represents a stage in the overall treatmentplan. Thus, five stages are depicted, as there are five appliances 100shown. The first three appliances 200, 201, and 202, respectively, mayhave a uniform elastic modulus chosen for a specific type of toothmovement. For example, appliances 200, 201 and 202 may be designed forpure translation, requiring a relatively high elastic modulus 111. Thus,the appliances are not shaded in the illustration. At stage 4, adifferent type of tooth movement, such as tipping, may be desiredrequiring a lower elastic modulus 110. Therefore, appliances 203 and 204may continue the series of differing shape or tooth geometries to createsuch movements, but the elastic modulus may differ from the priorappliances, 200, 201, and 202. Thus, these appliances are shaded in theillustration. The remainder of the treatment plan may feature a similarseries of appliances, including appliances with uniform elasticmoduluses which differ from the appliances immediately prior and/or anyappliances previously presented in the series. Likewise, such a seriesmay also include appliances' with combined variances, as describedabove.

[0055] Similarly, as shown in FIG. 12, a treatment plan may beprescribed with a series of appliances 100 differing in shape or toothgeometry, of which four stages are depicted, 210, 211, 212 and 213. Suchappliances may have any given elastic modulus that is suitable for theprescribed function. Likewise, such appliances may have internalvariance in elastic modulus, described previously, or may vary whollyfrom appliance to appliance throughout the prescribed treatment plan.However, if a patient were to discontinue usage of an appliance for anunprescribed period of treatment time, such as between stages two(appliance 211) and three (appliance 212) depicted in FIG. 12, thepatient's teeth may move slightly out of the planned tooth arrangement.Such a patient may be considered “off track” in which their currenttooth arrangement has diverted from the series of projected tootharrangements, creating an unprescribed tooth arrangement. Whenattempting to apply the next successive appliance 212, it may be toorigid to accommodate these slight differences. Therefore, a new moreflexible appliance 214 may be produced for this purpose and may beincorporated into the treatment plan. Such an appliance 214 may have thesame shape or tooth geometry as the next successive appliance 212, butit may have a lower elastic modulus 110, depicted by shading. Theincreased flexibility may allow the appliance 214 to conform to theunprescribed arrangement and reposition the teeth toward an arrangementthat the next successive appliance 212 may therefore fit. Such anappliance 214 may be used at any point in the series of successiveappliances.

[0056] As shown in FIG. 13, a series of incremental appliances, 300,301, 302, 303, 304 and 305, may be produced with differing elasticmoduluses, illustrated by variation in shading, to reposition teeth froman initial tooth arrangement to the next successive tooth arrangement ina progression of arrangements to the final arrangement. FIG. 13illustrates two steps in such a progression. A step or stage representsa change in shape or geometry of an appliance 100 to reposition theteeth into the next prescribed arrangement in a series. Therefore,appliances 300, 301, and 302 represent the first stage and have oneshape and 303, 304, and 305 represent the second stage and have adiffering shape. The appliances 300, 301, and 302 representing the firststage may vary in elastic moduluses from more flexible (appliance 300)to more rigid (appliance 302). The patient may begin the treatmentsequence with the more flexible appliance 300 of the first stage. Suchflexibility may allow an appliance with a substantially misalignedgeometry to fit over the patient's teeth and apply repositioning forces.As the teeth gradually move toward the desired arrangement, the patientmay progress to the next appliance 301 in the first stage. Thisappliance 301 may be more rigid than the prior appliance 300. Thepatient may continue through any number of appliances throughout astage. Upon completion of the stage, the patient may repeat the processin stage two, beginning with the more flexible appliance 303 andculminating with the more rigid appliance 305. The patient may thencontinue through any number of stages to the endpoint of treatment.

[0057] Such a system may provide a number of benefits. First, thevariance in elastic modulus throughout each step may allow for a largerstep or increment in tooth movement between each step than may beobtainable with consistent, rigid appliances. Such flexibility may allowthe appliance to fit over a tooth arrangement that is more misalignedwhile the increase in rigidity throughout each stage may providesufficient repositioning forces which may not be obtainable with highlyflexible appliances. These larger steps require fewer appliances in aseries to have a change in shape or geometry. Consequently, fewer moldsmay be required to form such appliances, which lowers cost and treatmenttime for the patient. In addition, if the patient were to become “offtrack” by suspending treatment, it may be possible for the patient toresume the prescribed treatment plan by reentering treatment at thestart of the step or stage in which the patient previously aborted. Thisappliance may be flexible enough to fit over the teeth in theunprescribed arrangement and gradually reposition the teeth throughoutthe stage as originally prescribed. This may also reduce cost andtreatment time since the production and fitting of a flexible “offtrack” appliance, as illustrated in FIG. 12, may be avoided.

[0058] The elastic modulus of an appliance or portions of an applianceof the present invention may be determined by a number of designfeatures, methods, materials and similar means. In a preferredembodiment, the appliance may be comprised of a polymeric shell which isheat formed over a mold of a patient's dentition. This is typicallyaccomplished by heating a thermoformable polymer material and applyingvacuum or pressure to form the polymer to the mold. Alternatively,reaction casting may be used to produce such an appliance. Hereinafter,description will pertain to thermoforming, however such concepts andtechniques may be equally applied to reaction casting or similar methodsand are not to limit the scope of the invention.

[0059] To produce an appliance with uniform elastic modulus, a polymersheet with a specific elastic modulus and thickness may be thermoformedover a mold and trimmed for patient use. Appliances with differinguniform elastic moduluses may be produced by altering one or more ofthree variables: 1) polymer type, 2) elastic modulus, 3) thickness. Toproduce an appliance with portions of differing elastic moduluses, anumber of techniques may be utilized. Referring to FIG. 14, portions ofpolymer sheeting 400 may be positioned over a mold 401 in designatedareas and thermoformed together into a final polymeric appliance. Eachportion of sheeting 400 may be chosen based on the three above mentionedvariables to provide a desired elastic modulus. Each portion of sheeting400 may then be positioned in the desired location for elastic moduluschanges throughout the finished appliance. In FIG. 14, three portionsare presented, a first sheet 402 placed over the right side molars, asecond sheet 403 placed over the left side molars and a third sheet 404placed over the remainder of the teeth. Sheets 402 and 403 are depictedas having differing elastic moduluses to each other and to sheet 404, asshown by shading gradations, however such sheets 402, 403, may beidentical. After thermoforming, a finished appliance may appear as thatillustrated in FIG. 2.

[0060] In addition, portions with differing elastic moduluses may becreated with the same polymer or different polymers material bylayering. Two layers of a polymer material bonded together may have ahigher or elastic modulus than a single layer of such material. Asillustrated in FIG. 15, a first sheet 405 may be placed over theincisors, canines and premolars of the mold 401 and a second sheet 406may be placed over the entire dentition. Each sheet may be the same ormay differ in terms of any or all of the above mentioned variables.After thermoforming, a finished appliance may also appear as thatillustrated in FIG. 2. In this case, the shell covering the molars iscomprised of one layer and the remainder of the appliance is comprisedof two layers formed into an integral appliance structure. Therefore,the portions covering the molars may have a lower elastic modulus,depending on the combination of materials, than the remaining portion.However, it is possible that a multi-layered structure may have a lowerelastic modulus than a single layered structure depending on the abovementioned variables., Thus, it may be appreciated that the describedlayering technique may provide a variety of moduluses and those statedexamples are not intended to limit the scope of the invention.

[0061] Similarly, portions with different elastic moduluses may becreated by a multi-step process of layering. Referring to FIG. 16, afirst sheet may be thermoformed over the entire dentition of a mold 401to form a base appliance 410. Portions desired to be of a differingelastic modulus 411, demarcated by a dashed line, may be cut and removedfrom the formed base appliance 410. A second sheet 412 may then bethermoformed over the entire dentition. This may result in a singlelayer of material in the portion of differing elastic modulus 411 and adouble layer of material in the remaining areas.

[0062] It may be appreciated that appliances with differing andgradually changing elastic moduluses may be created by any number ofproduction methods. For example, a base appliance 410 may be coated in aspecific area with one or more polymer solutions to “build up” a portionof the appliance for localized rigidity. Such a build-up may also begradual for a more gradual increase in rigidity. Likewise, a baseappliance 410 may be treated in specific areas with various chemicalagents to either increase or reduce localized rigidity. This may alsoinclude treatments involving temperature changes and other phasealtering methods. Similarly, such methods may be combined, including anyor all of the above described methods. Likewise, such methods may beutilized for appliances of uniform elastic modulus.

[0063] The fabrication process illustrated in FIG. 16 may be used toprepare a preferred laminated appliance structure 500, as illustrated inFIGS. 17 and 18. An inner layer 502 is formed from a relatively stiffpolymeric material and molded over a positive tooth model whichrepresents the desired appliance geometry. After the layer 502 isformed, it can be segmented into two or more sections which conform toindividual teeth or groups of teeth when the appliance is placed overthe patient's jaw. As shown in FIG. 17, gaps 504 may be formed betweenindividual sections 506, each of which conform to and receive anindividual tooth or group of teeth. An outer layer 508 is continuous inthe mesial-distal axis and covers all the segments 506 of the innerlayer 502. By providing an inner layer 502 having a higher stiffness,firm gripping or anchoring of the underlying teeth can be achieved.Moreover, by providing an outer layer 508 which is less stiff or morecompliant, ease of removing and replacing the appliance can besignificantly improved. Moreover, the stiffness or anchoring force canbe enhanced without having to concurrently modify the overall oreffective elasticity of the appliance which can be selected based on theclinical requirements of moving teeth. That is, the elasticity of theouter layer can be selected to provide an appropriate tooth movementforce while that of the inner layer can be chosen to enhance seatingcharacteristics over the teeth. In a specific embodiment, the complianceof the outer layer 508 could be varied along the mesial-distal axis inorder to provide for differing forces on the-teeth, as discussedgenerally above.

[0064] Although the foregoing invention has been described in somedetail by way of illustration and example, for purposes of clarity ofunderstanding, it will be obvious that various alternatives,modifications and equivalents may be used and the above descriptionshould not be taken as limiting in scope of the invention which isdefined by the appended claims.

What is claimed is:
 1. A dental appliance comprising: a shell havingcavities shaped to receive teeth, wherein at least a first portion ofthe shell is composed of a material having a first stiffness and asecond portion of the shell is composed of a material having a secondstiffness which differs from the first stiffness.
 2. An appliance as inclaim 1 , wherein the first portion differs in stiffness from the secondportion along a mesial-distal axis.
 3. An appliance as in claim 2 ,wherein the first portion receives one or more said teeth and the secondportion receives one or more said teeth.
 4. An appliance as in claim 1 ,wherein the first portion differs in stiffness from the second portionalong a facial-lingual axis.
 5. An appliance as in claim 4 , wherein thefirst portion partially covers an occlusional surface of a tooth and thesecond portion substantially covers the remainder of the tooth.
 6. Anappliance as in claim 1 , wherein the first portion differs in stiffnessfrom the second portion along a gingival-crown axis.
 7. An appliance asin claim 6 , wherein the first portion substantially covers anocclusional surface of a tooth and the second portion substantiallycovers the remainder of the tooth.
 8. An appliance as in claim 1 ,further comprising a third portion having a third stiffness whichdiffers from the first and second stiffnesses.
 9. An appliance as inclaim 8 , wherein the third portion is located adjacent to the firstportion or the second portion in a mesial-distal, facial-lingual orgingival-crown direction.
 10. An appliance as in claim 1 , wherein thefirst and second portions comprise layers in a laminar structure.
 11. Anappliance as in claim 10 , wherein at least one of the first and secondportions is continuous along a mesial-distal axis.
 12. An appliance asin claim 10 , wherein at least one of the first and second portions isdiscontinuous along a mesial-distal axis.
 13. An appliance as in claim10 , wherein the first portion comprises a discontinuous inner layer andthe second portion comprises a continuous outer layer and wherein atleast a portion of the inner layer is configured to engage individualteeth and the outer layer has a lower elastic modulus than the innerlayer.
 14. An appliance as in claim 1 , wherein the first portion andthe second portion have elastic moduluses in the range from 0.5 to 5GPa.
 15. An appliance as in claim 1 , wherein the first portion has afirst elastic modulus that differs from the second elastic modulus ofthe second portion by 25% to 600%.
 16. An appliance as in claim 1 ,wherein at least a region of the shell is comprised of a materialreinforced with a structure selected from the group consisting ofpieces, strips, wires, mesh, lattices, and networks.
 17. An appliance asin claim 16 , wherein the reinforced material comprises a polymer. 18.An appliance as in claim 16 , wherein the structure comprises a metal oralloy.
 19. An appliance as in claim 16 , wherein the structure comprisesa polymer.
 20. An appliance as in claim 1 , wherein at least a region ofthe shell comprises a metal or alloy.
 21. An appliance as in claim 1 ,wherein at least a region of the shell comprises of an interpenetratingnetwork.
 22. In a system for repositioning teeth from an initial tootharrangement to a final tooth arrangement, said system comprising aplurality of incremental position adjustment appliances comprisingshells having cavities shaped to receive and reposition teeth and whichare successively worn by a patient to move teeth from one arrangement toa successive arrangement, an improvement comprising: at least oneappliance including a shell wherein at least a first portion of theshell is composed of a material having a first stiffness and a secondportion of the shell has a second stiffness.
 23. In a system forrepositioning teeth from an initial tooth arrangement to a final tootharrangement, said system comprising a plurality of incremental positionadjustment appliances comprising shells having cavities shaped toreceive and reposition teeth and which are successively worn by apatient to move teeth from one arrangement to a successive arrangement,an improvement comprising: at least a first appliance including a shellhaving a portion with a first stiffness and at least a second appliancehaving a shell with a portion geometrically corresponding to that of thefirst appliance, wherein the portion of the second appliance has asecond stiffness which differs from the first stiffness.
 24. A system asin claim 23 , wherein the shells of the first and second appliances haveuniform but different stiffnesses over their entire tooth contact area.25. A system as in claim 24 , further comprising at least a thirdappliance having a stiffness which is uniform and different from thoseof the first or second appliance.
 26. A system as in claim 25 , whereinappliances with substantially similar stiffnesses are used forsubstantially similar tooth movements, wherein the tooth movements areselected from the group of tipping, translating, root uprighting,rotation, extrusion, intrusion and a combination of these.
 27. In asystem for repositioning teeth from an initial tooth arrangement to asuccessive tooth arrangement, said system comprising a plurality ofincremental position adjustment appliances comprising shells havingcavities shaped to receive and reposition teeth and which aresuccessively worn by a patient to move teeth from one arrangement to asuccessive arrangement, an improvement comprising: a first applianceincluding a shell having a shape which is the same as the shape of asecond appliance and having at least a portion with a stiffness which isdifferent from the stiffness of a corresponding portion in anotherappliance.
 28. A system as in claim 27 , further comprising a thirdappliance having a shape which is the same as the shape of the first andsecond appliances and having a stiffness in a portion which is differentfrom the stiffnesses of the corresponding portions of the first andsecond appliances, wherein the stiffnesses of the first, second, andthird appliances increase to provide incremental tooth movements as theappliances are successively worn.
 29. A method for producing a dentalappliance with varying stiffnesses, said method comprising: providing amold of dental features; forming a first layer of a material having afirst stiffness over a first portion of said mold; and forming a secondlayer of a material having a second stiffness over a second portion ofsaid mold; and laminating the first and second layers into an integralappliance.
 30. A method as in claim 29 , wherein the first portion ofthe mold and the second portion of the mold have no portions in common.31. A method as in claim 30 , wherein the forming steps occursimultaneously.
 32. A method as in claim 29 , wherein the first portionof the mold and the second portion of the mold at least partly overlap.33. A method as in claim 32 , wherein the forming steps occur insuccession.
 34. A method as in claim 33 , wherein further comprisingremoving material from the first layer prior to forming the secondlayer.
 35. A method as in claim 34 , wherein removing material comprisesseparating adjacent sections of the first layer so that said separatedsections conform to individual teeth or groups of teeth when theappliance is placed over teeth.
 36. A method as in claim 35 , whereinthe first layer is more stiff than the second layer.
 37. A method as inclaim 29 , wherein said forming steps are comprised of thermoforming,reaction casting, coating, treating, or altering.
 38. A method forrepositioning teeth, said method comprising: placing a first incrementalposition adjustment appliance over teeth in a patient's mouth, andsuccessively placing at least a second incremental position adjustmentappliance over the teeth in the patient's mouth, wherein the appliancescomprise polymeric shells having shaped cavities and wherein the shellsof the first and second appliances have the same shapes but haveportions which differ in stiffness between the first and secondappliances.
 39. A method as in claim 38 , wherein the stiffness of theshells of the first and second appliances are uniform over the entireshell.
 40. A method as in claim 38 , wherein the stiffnesses of theshells of the first and second appliances are non-uniform over theentire shell.
 41. A method for repositioning teeth, said methodcomprising: placing a first incremental position adjustment applianceover teeth in a patient's mouth, and successively placing at least asecond incremental position adjustment appliance over the teeth in thepatient's mouth, wherein the appliances comprise polymeric shells havingshaped cavities and wherein at least one of the first and secondappliance has a shell wherein at least a first portion of the shell hasa first stiffness and a second portion of the shell has a secondstiffness which differs from the first stiffness.
 42. A method as inclaim 41 , wherein both the shells of both the first and secondappliances having shells which at least a first portion of the shell hasa first stiffness and a second portion of the shell has a secondstiffness which differs from the first stiffness.
 43. A method as inclaim 38 , wherein the first and second portions of the shells of thefirst and second appliances are located in corresponding positions ofeach appliance and wherein the stiffness of the first portions of thefirst and second appliances differ from each other.
 44. A method as inclaim 43 , wherein the stiffness of the second portions of the first andsecond appliances differ from each other.
 45. A method as in claim 38 ,wherein the shells of the first and second appliances having the samegeometry.
 46. A method as in claim 38 , wherein the shells of the firstand second appliances have different geometries.